Devices provided for the extrusion of extrudable, although typically viscous, materials (e.g., caulking guns) usually suffer from the shortcoming that when the extruding force is stopped, the material tends to continue to be expressed through the orifice or nozzle of the device. This results from expansion forces developed in the material in opposition to compression forces applied to the material to cause it to be expressed. The extent to which such expansion forces are developed depends on a number of factors such as the size and shape of the orifice, the viscosity, compressibility, and elasticity of the material, the rate and amount of compression force applied, the temperature, frictional considerations, etc.
This phenomenon of post-compression expansion may be readily observed in most extrusion devices, from industrial plastics fabricating machines to the common, hand-operated caulking gun used by the home handyman. While the waste of material resulting from this phenomenon is often within tolerable limits and is therefore ignored, it is always a nuisance and it becomes troublesome and expensive in industrial applications where the expressed extrudate may be difficult to clean up or is too expensive to waste. The problem is especially troublesome in those industrial operations involving applicator guns operated by hand. It is to these that the present invention is particularly addressed.
Hand-operated caulking guns are very well known, e.g., U.S. Pats. Nos. 1,986,166; 2,420,203; 2,530,359; 2,561,825; 2,582,156; 2,602,570; 2,602,571; 2,732,102; and 2,786,604. Such guns are characterized by the presence of a canted anti-kickback dog, pawl or plate having a fixed pivot point and permitting free forward movement of the piston or plunger shaft assemblies, but continuously engaging the shaft to prevent any reverse motion of the shaft once its forward stroke ceases. It is known that permitting some reverse motion of the shaft will permit expansion of the caulking material and thereby tend to prevent dribbling from the nozzle. U.S. Pat. No. 2,815,151 describes a rather complicated caulking gun mechanism designed to permit a certain amount of desirable reverse motion of the plunger.